With the need for smaller sizes, lower weights and higher functionality in portable electronic devices such as smart phones, digital cameras and handheld game consoles, the development of high-performance batteries has been actively promoted in recent years, and demand for secondary batteries—which can be repeatedly used by charging—is growing rapidly. Lithium-ion secondary batteries in particular, because of their high energy density and high voltage, and moreover because they have no memory effect during charging and discharging, are the secondary batteries currently being most vigorously developed. Electrical car development is also proceeding apace as part of recent efforts to tackle environment problems, and an even higher level of performance is being demanded of the secondary batteries that serve as the power source in such vehicles.
Lithium-ion secondary cells have a structure in which a container houses a positive electrode and a negative electrode capable of intercalating and deintercalating lithium and a separator interposed between the electrodes, and is filled with an electrolyte solution (in the case of lithium-ion polymer secondary cells, a gel-like or completely solid electrolyte instead of a liquid electrolyte solution).
The positive electrode and negative electrode are generally produced by forming a composition which includes an active material capable of intercalating and deintercalating lithium, an electrically conductive material composed primarily of a carbon material, and a binder resin into a layer on a current collector such as copper foil or aluminum foil.
Carbon materials are widely used as negative electrode active materials. Recently, given the desire for further improvement in battery safety, active research and development has been carried out on negative electrode active materials having a high electric potential. For example, it is reported that titanium-containing oxides such as titanium oxide and lithium titanate are useful as such negative electrode active materials (Patent Documents 1 and 2).
However, because titanium-containing oxides have a low electrical conductivity compared with negative electrode active materials that use a carbon material such as graphite, in secondary batteries having a negative electrode active material layer that includes a titanium-containing oxide, the contact resistance between the negative electrode active materials and the contact resistance at the interface between the negative electrode active material and the current collector are high. As a result, the internal resistance and impedance of the secondary battery rise, making rapid charging and discharging at a large current impossible. Hence, in cases where a titanium-containing oxide is used as the negative electrode active material, measures taken to increase the conductivity of the negative electrode active material layer include adding a large amount of a conductive additive to the negative electrode active material layer, or coating a conductive material onto the surface of the negative electrode active material. A drawback in such cases is that the volume-based or weight-based capacity of the negative electrode active material layer declines in the degree to which material that does not contribute to the electrical capacity is added to the negative electrode active material layer.
Patent Document 3 describes art for producing secondary batteries in which, by placing an undercoat layer between a negative electrode active material layer and a current-collecting substrate and thereby lowering the resistance at the contact interface between the negative electrode active material layer and the current-collecting substrate, even in cases where a titanium-containing oxide is used in the negative electrode active material, a large amount of conductive additive is not added to the negative electrode active material layer, the internal resistance and impedance of the secondary cell are small, and the cell can be rapidly charged and discharged at a large current. Yet, in Patent Document 3, acetylene black is used as the conductive material in the undercoat layer, and the coating weight of the undercoat layer cannot be made sufficiently small. When one thinks of the undercoat layer as part of the electrode, this has the unfortunate result of reducing the electrode capacity.